The present invention is directed to new and useful heat shrinkable film formulations. One distinguishing feature of a shrink film is the film's ability, upon exposure to a certain temperature, to shrink or, if restrained from shrinking, to generate shrink tension within the film.
The manufacture of shrink films, as is well known in the art, may be generally accomplished by extrusion of the resinous materials which have been heated to their flow or melting point from an extrusion die in tubular or planar form. After a post extrusion quenching to cool, the extrudate is then reheated to its orientation temperature range. The orientation temperature range for a given film will vary with the different resinous polymers and blends thereof which comprise the film. However, the orientation temperature range may generally be stated to be above room temperature and below the melting point of the film.
The terms "oriented" or "orientation" are used herein to describe the process and resultant product characteristics obtained by stretching and immediately cooling a resinous polymeric material which has been heated to its orientation temperature range so as to revise the molecular configuration of the material by physical alignment of the molecules to improve mechanical properties of the film such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838-69 (reapproved 1975). When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is applied in two directions biaxial orientation results. Orientation is also herein used interchangeably with "heat shrinkability" with these terms designating a material which has been stretched and set by cooling at its stretched dimensions. An oriented (i.e., heat shrinkable) material will tend to return to its original unstretched dimensions when heated to an appropriate temperature below its melting temperature range.
Returning to the basic process for manufacturing the film as discussed above, it can be seen that the film once extruded and initially quenched to cool is then reheated to its orientation temperature range and oriented. The stretching to orient may be accomplished in many ways such as, for example, by "blown bubble" techniques or "tenter framing". These terms are well known to those in the art and refer to orientation steps whereby the material is stretched in the cross or transverse direction (TD) and in the longitudinal or machine direction (MD). After being stretched, the film is rapidly cooled to quench and thus set or lock-in the oriented molecular configuration.
After locking-in the oriented molecular configuration the film may then be stored in rolls and utilized to tightly package a variety of items. In this regard, the product to be packaged is first enclosed in the heat shrinkable material by heat sealing the shrink film to itself where necessary. Thereafter, the enclosed product is subjected to elevated temperatures by, for example, passing the product through a hot air or hot water tunnel. This causes the film to shrink around the product to produce a tight wrapping that closely conforms to the contour of the product.
The above general outline for manufacturing films is not meant to be all inclusive since this process is well known to those in the art. For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of these patents are hereby incorporated by reference.
Many variations on the above discussed general processing theme are available to those in the art depending upon the end use for which the film is to be put and the characteristics desired to be instilled in the film. For example, the molecules of the film may be cross-linked during processing to improve the films abuse resistance and other characteristics. Cross-linking and methods for cross-linking are well known in the art. Cross-linking may be accomplished by irradiating the film or, alternatively, may be accomplished chemically through the utilization of peroxides. Another possible processing variation is the application of a fine mist of silicone spray to the interior of the freshly extruded material to improve the further processability of the material. A method for accomplishing such internal application is disclosed in copending U.S. patent application Ser. No. 523,294 filed Aug. 15, 1983 which is a rule 1.62 continuation of U.S. application Ser. No. 289,018 filed in the U.S. Patent and Trademark Office on July 31, 1981, now abandoned, and hereby incorporated by reference.
The polyolefin family and, in particular, the polyethylene family of shrink films provides a wide range of physical and performance characteristics such as shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in linear dimension in a specified direction that a material undergoes when subjected to elevated temperatures while unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to tear apart), sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), and dimensional stability (the ability of the film to retain its original dimensions under different types of storage conditions). Film characteristics play an important role in the selection of a particular film and they differ for each type of packaging application and for each package. Consideration must be given to the product size, weight, shape, rigidity, number of product components, other packaging materials which may be used along with the film, and the type of packaging equipment available.
In view of the many above discussed physical characteristics which are associated with polyethylene films and in further view of the numerous applications with which these films have already been associated and those to which they may be applied in the future, it is readily discernible that the need for ever improving any or all of the above described physical characteristics of these films is great and, naturally, ongoing.